The Troll A platform is a condeep offshore natural gas platform in the Troll gas field off the west coast of Norway. It is the tallest and heaviest structure that has ever been moved to another position. The Troll platform was towed over 200 kilometers from Vats, in the northern part of Rogaland, to the Troll field, 80 kilometers north-west of Bergen. The Troll A platform has an overall height of 472 metres (1,549 ft), weighs 683,600 tons (1.2 million tons with ballast), standing on the sea floor 303 metres (994 feet) below the surface of the sea. The walls from the legs alone are over 1 metre thick made of steel reinforced concrete formed in one continuous pour, smoothly covering the great diameter of the legs. Troll A was built by Norwegian Contractors for Norske Shell, with base construction beginning in July 1991 at a cost of $650 million USD. The base and the deck were built separately, and were joined in 1995 while the base was partially submerged. The base is a Condeep gravity base structure built from reinforced concrete.